Apparatus for measuring a temperature of a substrate during a drilling operation

ABSTRACT

The present invention provides a system for recording a heat exchange event within a drilled hole wall of a substrate during a drilling process. In particular, the invention includes a thermal sensitive film placed in contact with a substrate which is to be drilled, which is capable of recording a thermal signature of the heat transferred during the drilling process. The result is a ring shaped signature whose thickness provides a direct correlation to drilling temperature. With these results, the integrity of the substrate surrounding the drilled hole can be inspected.

This application is a divisional of Ser. No. 09/017,741, filed on Feb.3, 1998.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to drilling operations, and morespecifically relates to a system and method for measuring thermal damageto a printed circuit board during a drilling operation.

2. Background Art

In the process of manufacturing printed circuit boards used in theelectronics industry, many holes of various sizes have to be drilledthrough printed circuit boards to accommodate electrical interconnectionof various layers of the printed circuit board and the electroniccomponents mounted on it. Such interconnections are typicallyimplemented with "plated through holes" (hereinafter, PTH's). As devicesizes decrease and boards become more densely populated, it becomes muchmore challenging to drill holes with a high degree of integrity. Becausehole sizes are relatively small, and multiple materials with differingproperties are utilized within the circuit board, a great deal of careand precision is required during such drilling operations.Unfortunately, failures caused by thermal damage during drillingoperations remain problematic because the damage is often difficult todetect and sometimes does not surface until a board failure occurs inthe field. Thermal damage to epoxy resins, which hold the layerstogether, and copper lands, which provide electrical contact pointswithin the circuit board, must therefore be guarded against duringdrilling operations of printed circuit boards.

The problem of ensuring drilled hole integrity is complicated by thefact that a typical drilling operation includes numerous variables, eachof which directly affects the integrity of the drilled hole. Forexample, drill bit geometry, circuit board materials and feed and speedparameters all contribute to the amount of heat generated in the drilledhole. Thus, before any change can be made to one of these and many othervariables, testing must be performed to ensure that the drilling processmeets the performance requirements of the circuit board. In general, iftoo much heat is generated within the drilled hole, damage to the nearbyportions of the circuit board could potentially lead to failure.

Present techniques for measuring hole quality with respect toplated-through holes (PTH's) require the time consuming step ofcarefully dissecting a drilled circuit board, and examining the damagearound the drilled hole, typically with a microscope or the like. Suchtechniques are both costly and time consuming. Therefore, any time achange of process, materials or the like is required, the cost ofmanufacturing a printed circuit board is increased.

A related problem involves damage resulting from drill bit wear. As adrill bit deteriorates, the drilled hole precision and integritylikewise deteriorates. Unfortunately, it is difficult to determine justwhen it is time to change the drill bit in a drilling operation.Changing the drill bit too soon results in additional costs resultingfrom bit replacement, and changing the bit too late potentially resultsin a circuit board malfunction.

Unfortunately, until now, there has been no inexpensive, simple andaccurate method of measuring drilled hole integrity in drillingoperations involving printed circuit boards and other bulk materials.This method could help reduce the cost of manufacturing printed circuitboards.

SUMMARY OF THE INVENTION

The present invention provides a system and method for determiningdrilled hole integrity by recording heat generation within a drilledhole during a drilling operation. In particular, the invention providesa thermal sensitive material that is in contact with a substrate duringdrilling, wherein the thermal sensitive material is capable of recordinga signature that measures the heat generated within the hole wallsduring a drilling operation. The thermal sensitive material is drilledalong with the substrate. The thermal sensitive material may comprisethermal paper, acetate, and/or any other film-like material that iscapable of recording thermal changes on a surface to which the film hasmade contact.

The method according to this invention comprises the steps of placing athermal recording film on a surface that is to be penetrated with adrill bit, drilling a hole through both the film and the surface suchthat a thermal signature is recorded on the film proximate the hole,examining the size of the thermal signature and determining a relativeor absolute amount of heat generated in the hole. The film may also be"sandwiched" between a plurality of layered substrates.

It is therefore an advantage of the present invention to provide a meansfor measuring drilling temperature, and to assess the hole quality withthe use of a thermal sensitive material.

It is a further advantage of the present invention to provide a systemand method for checking the integrity of holes drilled within a printedcircuit board.

It is a further advantage of the present invention to provide a thermalsensitive film that contacts a substrate in order to analyze or monitordrilled hole integrity.

It is a further advantage of the present invention to provide a systemand method for calculating drilling temperature based upon the size of aring formed on a thermal sensitive film in contact with a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent from the following detailed description of anexemplary embodiment therefore, as illustrated in the accompanyingdrawings, in which:

FIG. 1 depicts an exploded view of a printed circuit board;

FIG. 2 depicts a printed circuit board in contact with a thermalsensitive film in accordance with a preferred embodiment of the presentinvention;

FIG. 3 depicts a cross-sectional view of a printed circuit board incontact with a thermal sensitive paper and a drill bit penetratingtherethrough in accordance with a preferred embodiment of the presentinvention;

FIG. 4 depicts a thermal signature left on a piece of thermal sensitivefilm in accordance with a preferred embodiment of the present invention;

FIG. 5 depicts a top view of a printed circuit board showing thesignature left on the thermal sensitive film after various drillingoperations have occurred in accordance with a preferred embodiment ofthe present invention; and

FIG. 6 depicts a circuit land having a hole bored therethrough.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 depicts an exploded view of a substrate such as a printed circuitboard 10. The circuit board comprises a first dielectric layer 19, powerplane 18, a second dielectric layer 16, a circuit layer 14, and a thirddielectric layer 12. It is recognized that the actual number of layerswithin a typical circuit board may vary and the embodiment described inFIG. 1 is used primarily for the purpose of an example. Also shown inFIG. 1 is a vertical line 20 that defines a drilled hole that is typicalof those required in the fabrication of printed circuit boards. Inparticular, drilled holes are required such that electrical signals canbe delivered from various layers of the printed circuit board 10. Oncethe drilled hole is formed, vertical circuit paths or plated-throughholes (PTH's) can be utilized to connect circuits on different layers tocomponents mounted on the board. For instance, DC voltage from the powerplane 18 may be delivered to components by one PTH and signal logic fromcircuit layer 14 may be delivered to components by another PTH. Holes inprinted circuit boards are typically on the order of 0.25-1.0millimeters in diameter and may be utilized for electrical,electrical/mechanical, or any other purpose required on the circuitboard. In fabricating a printed circuit board, the layers are connectedtogether typically with an epoxy resin or the like, and then thenecessary holes are drilled through the circuit board and then the holesare plated with copper.

It should be recognized that while this preferred embodiment is directedat printed circuit boards, the invention can be applied or utilized inany operation where a substrate or bulk material is being drilled. Itshould likewise be recognized that the term "drilling operation" is tobe interpreted broadly as any hole boring operation, including, but notlimited to, laser drilling, punching, milling, etc.

FIG. 2 depicts the printed circuit board 10 of FIG. 1 with a thermalsensitive film 22 placeable on the surface of the circuit board 10.Pursuant to this invention, thermal sensitive film 22 may be placed oneither surface of the circuit board 10, or between any of the layers ofa stack of circuit boards and process materials. Also shown in FIG. 2 isa drill 24 which operates along a vertical axis to bore holes throughthermal sensitive film 22 and then through circuit board 10. When thefilm 22 is in contact with the circuit board 10, information regardingheat transfer caused during a drilling operation can be obtained. Thus,the heat transfer "event" caused by the work and friction of the drillbit drilling into the circuit board is captured on the film 22.

To capture the heat exchange event, the material used for thermalsensitive film 22 may be infrared sensitive so that it changes colorwhen it is exposed to heat. Examples include thermal facsimile paper,overhead projection film and infrared photographic film. The materialcan also be made of polymers which soften when exposed to heat. Examplesinclude thin transparent plastic sheets such as acetate, polyethylene,epoxy and polyamide. It should be recognized however, that any materialor combination of materials capable of recording a heat transfer eventcould be utilized within the scope of this invention. As noted, thetechniques described herein provide for the use of the film 22 to bepositioned as an entry, backer or interleaf during the drillingoperation of the circuit board. To obtain the best result, it has beenfound that using the film as an interleaf within a stack of parts orlayers will provide the best thermal signature. However, it isrecognized that other placements will still yield excellent results.

FIG. 3 depicts a cross-sectional view of a printed circuit board 10 witha thermal sensitive film 22 affixed on top thereof and a drill 24penetrating therethrough. The thermal sensitive film 22, according tothis invention, records a thermal signature of the drilling process. Inthis preferred embodiment, the thermal signature appears as a ringformed around the hole, after both the film 22 and circuit board 10 aredrilled. The ring formed on any infrared materials is essentially aphotographic image recording the heating event. A ring formed on aplastic type material is essentially a refractive distortion due toheating, softening and material flow. In the case of FIG. 3 where film22 is a piece of thermal sensitive paper, it can be seen that theheating event is captured as heat flows along gradients 26 from thewalls of the drilled hole through the circuit board and up to thesurface, where the event is recorded on the paper.

FIG. 4 depicts the sheet of film 22 with a thermal signature recordedthereon in the form of a ring 34 surrounding drilled hole 32. Becausedrilled hole quality can be directly correlated with drillingtemperature, the thickness of the ring 36 can be utilized to provide ameasurement of the drilling temperature, and therefore drilled holequality. Moreover, the size of the ring could be calibrated to measureactual drilling temperatures with birefringence or hot contact probestechniques. Thus, measurement could be absolute, relative forcomparison, quantitative or qualitative.

In general, for a given drilling time the temperature rise of theheating event caused by the drilling operation will be substantiallyproportional to the ring thickness 36. Thus, temperature may berepresented mathematically as:

    ΔT=αR,

where ΔT is the temperature rise, R is the ring thickness, and αrepresents a proportionality constant of the specific system. Inparticular, α is dependent upon system variables such as board thermalconductivity and film thermal sensitivity etc. α could be calculatedusing calibration techniques, or by using a foreign device to measuredrill hole temperature, so that actual temperatures can be calculatedfrom the ring size. Alternatively, comparative temperature informationcan be collected by comparing the ring size of multiple drillingoperations.

To further enhance the usefulness of this system and method, markings,such as concentric circles 3 5, can be included on the film 22 toprovide indicators. In this manner, the signature can be easilyinspected to determine if the event fell within the necessary qualityparameters to ensure integrity of the circuit board.

FIG. 5 depicts a first 28 and a second 30 series of holes and theirrespective thermal signatures as recorded on a sheet of thermalsensitive film 22. As can be seen, the first set of holes 28 left alarger signature as compared to these second set of holes 30. Ittherefore follows that the drilling temperature for the first set ofholes 28 was greater than for the second set of holes 30. With thisknowledge, quick and inexpensive decisions can be made regarding theefficacy of the drilling operation. Because this information is soreadily obtainable, it is very easy for the drilling process to bealtered without doing a detailed study of the circuit board, therebysaving both time and money.

Finally, FIG. 6 depicts a circuit land 38 which is essentially acircular conductive region through which a hole 32 can be drilled.Knowing the amount of the heat distributed radially outward from thehole in the printed circuit board, choices regarding materials, processparameters and the like can be made such that the integrity of this land38 will be kept intact. As can be seen, during a drilling operation, aheat-affected zone 40 with a radius 42 is created that is larger thanthe land 38, and therefore can potentially cause the circuit land 38 tofail. By knowing that the heat-affected zone 40 extends outside thediameter of the land 38, which would be calculated directly from thethermal signature recorded on a thermal sensitive film 22, potentialfailures can be identified ahead of time. Thus in this case, it may bedesirable to find a way to reduce the heat-affected zone, either bychanging the substrate materials, drill bit geometry or processparameters involved.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention should not be limited to the specific constructionsand arrangements shown and described, since various other modificationsmay occur to those ordinarily skilled in the art.

We claim:
 1. An apparatus for measuring a temperature of a substrateduring a boring operation, said apparatus comprising a film that is incontact with said substrate on a surface that is to be bored, whereinsaid film comprises a thermal sensitive material capable of recordingradial temperature changes occurring around a bored hole.
 2. Thetemperature measuring apparatus of claim 1 wherein said boring operationutilizes a drill.
 3. The temperature measuring apparatus of claim 1wherein said boring operation utilizes a laser.
 4. The temperaturemeasuring apparatus of claim 1 wherein said film comprises thermalpaper.
 5. The temperature measuring apparatus of claim 1 wherein saidfilm comprises thin transparent plastic.
 6. The temperature measuringapparatus of claim 1 wherein said substrate comprises a circuit board.7. The temperature measuring apparatus of claim 6 wherein said circuitboard includes a plurality of layered surfaces.
 8. The temperaturemeasuring apparatus of claim 1 wherein said radial temperature changesare recorded as a ring around the bored hole.
 9. The temperaturemeasuring apparatus of claim 8 wherein a thickness of said ring directlycorrelates to a maximum temperature in the bored hole at the time ofboring.
 10. The temperature measuring apparatus of claim 1 wherein saidfilm is placeable between two surfaces.
 11. A substrate having a thermalrecording mechanism for recording temperature change proximate an innerwall of a drilled hole during a drilling operation, wherein the thermalrecording mechanism comprises a film in contact with said substrate on asurface to be drilled, said film capable of recording radial temperaturechanges around the drilled hole.
 12. The substrate of claim 11 whereinsaid film comprises thermal paper.
 13. The substrate of claim 11 whereinsaid film comprises thin transparent plastic.
 14. The substrate of claim11 wherein said radial temperature changes are recorded on the film as avisible ring around the drilled hole.
 15. The substrate of claim 11wherein said substrate comprises a circuit board.